Field emission of electrons into vacuum from suitable cathode materials is currently the most promising source of electrons for a variety of vacuum devices. These devices include flat panel displays, klystrons and traveling wave tubes used in microwave power amplifiers, ion guns, electron beam lithography, high energy accelerators, free electron lasers, and electron microscopes and microprobes. A most promising application is the use of field emitters in thin, matrix-addressed flat panel displays. See, for example, the December 1991 issue of Semiconductor International, p. 46; C. A. Spindt et at., IEEE Transactions on Electron Devices, vol. 38, p. 2355 (1991); I. Brodie and C. A. Spindt, Advances in Electronics and Electron Physics, edited by P. W. Hawkes, vol. 83 pp. 75-87 (1992); and J. A. Costellano, Handbook of Display Technology, Academic Press, New York, pp. 254 (1992).
A typical field emission device comprises a cathode including a plurality of field emitter tips and an anode spaced from the cathode. A voltage applied between the anode and cathode induces the emission of electrons towards the anode.
A conventional flat panel field emission display (FED) comprises a flat vacuum cell having a matrix array of microscopic field emitters formed on a cathode of the cell (the back plate) and a phosphor coated anode on a transparent front plate. Between cathode and anode is a conductive element called a grid or gate. The cathodes and gates are typically skewed strips (usually perpendicular) whose crossovers define pixels for the display. A given pixel is activated by applying voltage between the cathode conductor strip and the gate conductor. A more positive voltage is applied to the anode in order to impart a relatively high energy (400-3,000 eV) to the emitted electrons. See for example, U.S. Pat. Nos. 4,940,916; 5,129,850; 5, 138,237 and 5,283,500, each of which is incorporated herein by reference.
Ideally, the cathode materials useful for field emission devices should have the following characteristics:
(i) The emission current is advantageously voltage controllable, preferable with drive voltages in a range obtainable from off-the-shelf integrated circuits. For typical device dimensions (1 .mu.m gate-to-cathode spacing), a cathode that emits at fields of 25 V/.mu.m or less is suitable for typical CMOS circuitry. PA1 (ii) The emitting current density is advantageously in the range of 0.1-1 mA/mm.sup.2 for flat panel display applications. PA1 (iii) The emission characteristics are advantageously reproducible from one source to another, and advantageously they are stable over a very long period of time (tens of thousands of hours). PA1 (iv) The emission fluctuation (noise) is advantageously small so as not to limit device performance. PA1 (v) The cathode is advantageously resistant to unwanted occurrences in the vacuum environment, such as ion bombardment, chemical reaction with residual gases, temperature extremes, and arcing; and PA1 (vi) The cathode is advantageously inexpensive to manufacture, without highly critical processes, and it is adaptable to a wide variety of applications.
Previous electron emitters were typically made of metal (such as Mo) or semiconductor (such as Si) with sharp tips in nanometer sizes. Reasonable emission characteristics with stability and reproducibility necessary for practical applications have been demonstrated. However, the control voltage required for emission from these materials is relatively high (around 100 V) because of their high work functions. The high voltage operation increases the damaging instabilities due to ion bombardment and surface diffusion on the emitter tips and necessitates high power densities from an external source. The fabrication of uniform sharp tips is difficult, tedious and expensive, especially over a large area. In addition, the vulnerability of these materials to ion bombardment, chemically active species and temperature extremes is a serious concern.
Diamond is a desirable material for field emitters because of its negative or low electron affinity and robust mechanical and chemical properties. Field emission devices employing diamond field emitters are disclosed, for example, in U.S. Pat. Nos. 5,129,850 and 5,138,237 and in Okano et al., Appl. Phys. Lett., vol. 64, p. 2742 (1994), all of which are incorporated herein by reference. Flat panel displays which can employ diamond emitters are disclosed in co-pending U.S. patent application Ser. No. 08/220,077 filed by Eom et al on Mar. 30, 1994, U.S. patent applications Ser. No. 08/299,674 and Ser. No. 08/299,470, both filed by Jin et al. on Aug. 31, 1994, and U.S. patent application Ser. No. 08/311,458 and 08/332,179, both filed by Jin et al. on Oct. 31, 1994, Ser. Nos. 08/361616 filed on Dec. 22, 1994, and Ser. No. 08/381375 filed on Jan. 31, 1995.
Diamond offers substantial advantages as low-voltage field emitters, especially diamond in the form of ultra fine particles or islands. These particles or islands can be made to exhibit sharp, protruding crystallographic edges and corners desirable for the concentration of an electric field. One of the most critical preparation steps for ensuring low-voltage field emission is the chemical bonding of the diamond particles or islands onto the surface of cathode conductor for good electrical contact. Experimental results teach that without strong bonding and associated good electrical contact, low-voltage field emission from diamond is not possible.
In the use of ultra fine or nanometer-type diamond particles, such as those disclosed in application Ser. Nos. 08/361616 and Ser. No. 08/381375, a good adhesion of the particles to the conductive substrate (and a desirable hydrogen termination of diamond surface) can be achieved by high-temperature heat treatment of the particles on the substrate in hydrogen plasma, typically at 300.degree.-1000.degree. C. While adequate emission characteristics can be obtained by the plasma heat treatment even below about 500.degree. C., further improved properties are generally achieved by higher temperature processing. However, other device components in a field emission display should not be exposed to a higher temperature processing. For example, the glass substrate desirably has a low melting point of about 550.degree. C. or below for the purpose of ease of vacuum sealing when the FED assembly is completed. This places an undue upper limit in the plasma heat treatment temperature and hence restricts the full utilization of the best attainable field emission characteristics from the diamond particles.
In the use of diamond islands such as are deposited by CVD (chemical vapor deposition) processing, it is also noted that better-quality diamond islands with desirably sharp crystallographic facets and corners, good chemical bonding, and good electrical contact to the conductor substrate, are generally obtained by CVD processing at temperatures higher than about 700.degree. C. Again, because of the restrictions in the maximum exposable temperature for the glass substrate and other components, it is difficult to obtain the best field emission characteristics of CVD diamond islands by higher temperature processing.